A V6 engine is a V engine with six cylinders mounted on the crankshaft in two banks of three cylinders, usually set at a 60 or 90 degree angle to each other. The V6 is one of the most compact engine configurations, usually ranging from 2.0 L to 4.3 L displacement, and it is shorter than the inline 4. Because of its short length, the V6 fits well in the widely used transverse engine front-wheel drive layout.
Recent forced induction V6 engines have delivered horsepower and torque output comparable to contemporary larger displacement, naturally aspirated V8 engines, while reducing fuel consumption and emissions, such as the Volkswagen Group's 3.0 TFSI which is supercharged and directly injected, and Ford Motor Company's turbocharged and directly injected EcoBoost V6, both of which have been compared to Volkswagen's 4.2 V8 engine.
The first series-production V6 was introduced by Lancia in 1950 with the Lancia Aurelia model. Lancia sought a smoother and more powerful engine that would fit into an existing narrow engine bay. Lancia engineer Francesco De Virgilio began analyzing the vibration of alternative V-angles for a V6 engine in 1943. He found that a V6 with its cylinders positioned at a 60À V-angle could be made uniquely smooth-running in comparison with other possible V-angles. There was resistance to his conclusion because the V6 was a virtually unknown engine type in the 1950s. His design featured four main bearings and six crankpins, resulting in evenly spaced firing intervals and low vibrations.
Straight engines with an odd number of cylinders are inherently unbalanced because there is always an odd number of pistons moving in one direction while a different number move the opposite direction. This causes an end-to-end rocking motion at crankshaft speed in a straight-three engine. V6 designs will behave like two unbalanced three-cylinder engines running on the same crankshaft unless steps are taken to mitigate it, for instance by using offset journals or flying arms on the crankshaft or a counter-rotating balance shaft.
In the V6 with 120À between banks, pairs of connecting rods can share a single crank pin, but the two cylinder banks run like two inline 3s, both having an end-to-end rocking couple. Unlike in a V8 engine with a crossplane crankshaft, the vibrations from one bank do not cancel the vibrations from the other, so a rotating balancing shaft is required to compensate for the primary vibrations. Because the 120À V6 is nearly as wide as a 180À flat-6 but is not nearly as smooth, and can be more expensive if a balancing shaft is added, this configuration is seldom seen in production engines.
Six-cylinder designs are also more suitable for larger displacement engines than four-cylinder ones because power strokes of pistons overlap. In a four-cylinder engine, only one piston is on a power stroke at any given time. Each piston comes to a complete stop and reverses direction before the next one starts its power stroke, which results in a gap between power strokes and annoying harshness, especially at lower revolutions. In a six-cylinder engine (other than odd-firing V6s), the next piston starts its power stroke 60À before the previous one finishes, which results in smoother delivery of power to the flywheel. In addition, because inertial forces are proportional to piston displacement, high-speed six-cylinder engines will suffer less stress and vibration per piston than an equal displacement engine with fewer cylinders.